1. Field of the Invention
This invention relates in general to semiconductor integrated circuit fabrication, and more particularly to a modified apparatus including a cup-type device for containing the developer, so as to improve the uniformity of the photoresist development process.
2. Description of Related Art
In today's semiconductor integrated circuit (IC) fabrication, designed mask patterns are transferred to the semiconductor substrate by a photolithography process. Since the precision of pattern transference determines the characteristics of the product device, the photolithography process is one of the most important steps of the entire process. Usually, a typical photolithography process includes the steps of photoresist coating, exposure, development, and photoresist removal. First, a photoresist is coated on a semiconductor substrate. An exposure step is applied using a photomask containing the desired circuit pattern to induce a photochemical reaction in an area of the photoresist not shielded by the photomask pattern. The exposed photoresist is then subjected to a development step. Depending upon the chemical nature of the photoresist material, the exposed areas may be rendered more soluble in some developing solvent than the unexposed areas, thereby producing a positive-tone image of the photomask. Conversely, the exposed areas may be rendered less soluble producing a negative-tone image of the photomask. The net effect of this process is to produce a three-dimensional relief image in the photoresist that is a replication of the distinction between the opaque and transparent areas on the photomask.
All of the parameters in performing the development step should be precisely controlled to prevent the unexposed areas of the photoresist from being attacked by the developer. The major factors effecting this process are known as follows: developing time, concentration and temperature of the developer. An appropriate developing time is chosen in coordination with other process parameters. Basically, the developing time should be long enough to completely dissolve the photoresist. However, the development step cannot be carried out too long since it results in a worse line width tolerance in the photoresist pattern.
The developer concentration has a significant influence on the development step. As the concentration of the developer is increased, the developing time is reduced; thereby the throughput of IC fabrication is improved. However, too short a developing time will degrade the resolution on the resultant photoresist pattern. Therefore, an appropriate concentration of the developer is chosen to comprise both the fabricating throughput and the pattern resolution. The developing temperature is usually maintained in a range near room temperature when performing the development step.
There are several kinds of development schemes currently being used by IC manufacturing plants. In a normal in-line process, a spray/puddle scheme is utilized to perform the photoresist development. The whole process can be divided into three stages. First, a semiconductor wafer that has been exposed is put on a holder. By using a nozzle, a developer is sprayed onto the surface of the semiconductor wafer. Next, the holder is shaken to help the exposed areas of the photoresist being dissolved by the developer. At the third stage, the holder is rotated to draw out the developer. However, since the developer is not sprayed over the semiconductor wafer uniformly this prior art development scheme generally results in improper pattern transference at the boundary area of the semiconductor wafer. This produces in poor device characteristics. As the diameter of the semiconductor wafer is continuously increased, the problem worsens.
FIG. 1 shows a cross-sectional view of a prior art photoresist development apparatus. As shown in the drawing, a development apparatus primarily consists of a holder for holding a wafer and a nozzle 14 for supplying the developer to the wafer. The holder at least includes a wafer chuck 10 and a vertical spindle 12. When executing the developing process, a semiconductor wafer 20 coated with a photoresist that has been exposed is first held on the wafer chuck 10. By using the nozzle 14, the developer is then sprayed onto the surface of the photoresist to start the developing reaction. Thus, the photoresist pattern 22 is formed on the semiconductor wafer 20.
However, the above-mentioned prior art development apparatus has several drawbacks. First, a certain amount of the developer will run off because there is no obstacle at the boundary area of the semiconductor wafer 20. Therefore, only a limited percent of the sprayed developer will remain on the semiconductor wafer 20 as a result of surface tension, which forms a thin liquid layer 25 for the photoresist developing reaction. Since it is difficult to retain a sufficient amount of the developer on the semiconductor wafer 20, the developer becomes saturated in the middle of the process and then the developing reaction stops. Second, since the developer is supplied by utilizing the nozzle 14 bubbles are easily formed in the liquid layer 25. This will slow down the developing reaction. Furthermore, as the developer remains on the semiconductor wafer 20 because of surface tension, the thickness distribution of the liquid layer 25 is not uniform. The thickness of a portion of liquid layer 25 near the boundary area of the semiconductor wafer 20 (i.e., the dashed line area indicated by a symbol II) is smaller than that of a portion of liquid layer 25 in the central area of the semiconductor wafer 20. Hence, the developing speed of the former is less than that of the latter. This results in the improper pattern transference at the boundary area of the semiconductor wafer 20, such as not completely transferring the pattern to the photoresist or having a line width that is wider than it should be. As the diameter of the semiconductor wafer 20 grows, the problem becomes even worse.
FIG. 2 is a partially magnified view of FIG. 1 especially showing the area indicated by the symbol II. The non-uniformity of distribution of the developer on the semiconductor wafer 20 and the problem of improper pattern transference are now more apparent in FIG. 2. As shown in the drawing, an exemplary photoresist pattern with lines (t.sub.1) and spaces (t.sub.2) of equal extent is provided. In the central area of the semiconductor wafer 20 (the left portion of FIG. 2), a photoresist pattern 22b is formed with lines and spaces of equal extent, i.e., t.sub.1 =t.sub.2. But in the area near the boundary of the semiconductor wafer 20 (the right portion of FIG. 2), a photoresist pattern 22a is formed with lines wider than spaces, i.e., t.sub.1 &gt;t.sub.2 due to inadequate development.